In conventional valve lift controllers, several types of actuators are used to linearly drive a shaft of a changing mechanism which controls a lift amount of a valve based on a position of the shaft in its axial direction. For example, an actuator is described in US 2004-0083997A1 (JP 2004-150332A) which converts, by means of a reduction mechanism and a cam mechanism, a rotational driving force of a motor unit into a linear driving force and applies the linear driving force to the shaft of the changing mechanism.
However, the conventional actuator has to use the reduction mechanism in combination with the cam mechanism to make the linear driving force larger. It is therefore difficult to design the actuator to be small. Thus, positions where the actuator can be installed are limited.
The inventors of the present invention have studied a structure of a feed screw mechanism which converts a rotational movement of a rotation spindle to a linear movement of a screwed shaft. The feed screw mechanism can generate a strong linear driving force by means of a simple structure in which the rotation spindle and the screwed shaft are coaxially connected directly or indirectly. An actuator with the feed screw mechanism therefore can be designed to be smaller than the actuator with the reduction mechanism and the cam mechanism.
According to further studies of the inventors on the feed screw mechanism, a problem occurs when the feed screw mechanism is located between the changing mechanism and an electric power distributor for distributing electric power to the motor unit. If the motor unit suddenly increases a thrust force applied to the screwed shaft in a direction toward the changing mechanism, the rotation spindle bumps into the electric power distributor by receiving a strong thrust resistance force toward a direction opposite to the changing mechanism (i.e. toward the electric power distributor). Since such a bump causes breakdown or malfunction of an electric circuit in the electric power distributor, it is better to avoid the bump to improve endurance of the actuator.